During postabsorptive conditions skeletal muscle predominately oxidizes FFA and has a high rate of uptake of plasma FFA. An impairment of muscle FFA utilization could therefore be an important factor in partitioning FFA metabolism toward esterification and storage. Recent studies using whole body indirect calorimetry chambers suggest that such a defect is present in obesity. The overall goal of this project is to determine whether skeletal muscle FFA utilization during postabsorptive conditions is reduced in obese individuals and whether this defect is improved by weight loss. In particular, we will test the hypothesis that impaired postabsorptive FFA utilization by muscle is an integral component of the visceral obesity/insulin resistance syndrome. I[unreadable]g balance methods will be used to measure FFA uptake, based on fractional extraction of labelled oleate, and limb indirect calorimetry will be used to determine FFA oxidation. Systemic FFA metabolism will also be measured. A euglycemic hyperinsulinemic clamp will be performed to determine insulin sensitivity. Vastus lateralis muscle, obtained by percutaneous muscle biopsy, will be analyzed for fiber type distribution, mitochondrial oxidative potential and content of key steps in FFA metabolism (e.g. cytosolic fatty acid binding protein, CPT, LpL, and beta-hydroxy acyl dehydrogenase). Body composition will be determined by dual-energy x-ray absorptiometry (total body adiposity) and abdominal computed tomography (to measure visceral fat). Physical fitness (VO2 max) will be determined as an potential co-variable with regard to muscle FFA metabolism. Based on our preliminary data in 17 women, a sample size of 60 men and women, comprising a continuum of body mass index from 20 to 40 kg/m2, will be adequate power to test this hypothesis. In order to determine the effects of weight loss, obese subjects will be studied a second time, after completing a 4 month weight loss program designed to achieve a 15 kg weight loss. Important new information will be obtained regarding whether visceral obesity is associated with decreased postabsorptive FFA utilization in muscle, a potentially important factor in the pathogenesis of obesity.